With the increasing sophistication of scientific testing and analysis, the requirements for uncontaminated water for laboratory use has likewise increased. The purity of water is graded according to various standards, and generally accepted standards are published in the Annual Book of ASTM Standards, Water and Environmental Technology, Standard Specifications for Reagent Water, Vol. 11.01, 1996. Of current interest is the ASTM Type 1 Reagent Grade Water Standard which specifies the highest purity water which is used for high performance liquid chromatography (HPLC), gas chromatography, trace metal analysis by atomic absorption spectrometry, tissue culture, etc.
The majority of impurities in potable water are in the form of disassociated ionized inorganic mineral salts which carry an electrical charge; therefore, the presence of ionic contaminants in water is directly related to the electrical conductivity or resistivity of the water. Conductivity and its reciprocal, resistivity, are therefore standard variables by which the ionic purity of water is measured. Specific resistance is measured in units of ohm-cm at a compensated reference temperature of 25.degree. C. Absolutely pure water has a theoretical specific resistance of 18.17 megohm-cm. (As will be appreciated, most resistivity meters read to 18.3 megohm-cm as an artifact of earlier theoretical R-25 calculations.)
Organic contaminants do not typically contain an ionic charge. Therefore, they cannot be detected by specific resistance measurements. To determine the organic content of water and other materials, TOC, HPLC, GC and GC/MS and other special instruments are used to detect the presence of low levels of specific organic compounds. Consequently, water used in these analyses should contain no more than a few ppb of total organic compounds.
Therefore, there is a continuing need to provide reagent grade water suitable for the most demanding laboratory protocols requiring low TOC reagents. Reagent grade water with lower TOC are or will be required to meet the increasingly lower detection level demands for HPLC, gas chromatography with mass spectrometry detection (GC-MS), TOC analysis and other emerging analytical instrument technologies. There is a further need for water particularly suited for reagent use in EPA test methods for detection of volatile organic compounds (VOC) in drinking water, as well as water suitable for routine inorganic laboratory protocols.